How would you like to live on an active volcano? Surprisingly, a lot of people are living on or near active volcanoes, and many more live near volcanoes that are currently considered to be “dormant”. Are they crazy? Maybe some are, but not all volcanoes erupt explosively; for example, the type of volcano that forms the Hawaiian Islands is a type that erupts effusively, with lava running down the sides (flanks) of the volcano.

In the previous chapter on igneous rocks, you learned about the concept of partial melting, and in the chapter on plate tectonics you learned about the conditions necessary for mantle rocks to melt; we will review these concepts in this section.

Refer to Figure 9.4 to help answer the questions. The exercises that follow the use of Google Earth. For each question (or set of questions) paste the location that is given into the “Search” box. When finding your locations in Google Earth, be sure to zoom out to higher eye elevations in order to see all of the important features of each area.

Once magma is generated by one of the mechanisms mentioned earlier (increased temperature, decreased pressure, or by adding water), the magma rises upward through the surrounding rock mainly through pre-existing fractures in the brittle lithosphere. A lot of magma stops rising upward through the continental crust because it has encountered an area in the crust that has the same density as the magma.

The questions in this exercise demonstrate the control that tectonic setting has on the type of magma produced. For the Google Earth questions, copy and paste the latitude and longitude coordinates into the search bar (or just type them in).

In the chapter on igneous rocks, you learned that the igneous rock classification is in part based on the mineral content of the rock. For example, ultramafic rocks are igneous rocks composed primarily of olivine and a lesser amount of calcium-rich plagioclase and pyroxene, whereas quartz, muscovite and potassium feldspar are the typical minerals found in felsic rocks.

The following questions address what factors control how fast a magma or lava can flow. The resistance to flow (viscosity) depends primarily on the magma or lava composition but is also affected by temperature.

The size, shape, and eruptive style of any volcano ultimately depend on the magma composition. We will focus mainly on mafic and felsic magmas as intermediate magmas have properties that are intermediate between these two types, and ignore the ultramafic magma as this type no longer forms. As mentioned earlier, mafic magmas are lower in silica, causing low viscosity. As mafic magma erupts on to the surface through a central vent, the magma will spread out quite easily due to its low viscosity.

The composition of magma or lava may also control what type of volcanic features or landforms are seen on the earth’s surface. In this exercise, you will use Google Earth to identify these landforms. Figures 9.9 through 9.11 may help you in this section. For the Google Earth questions, copy and paste the latitude and longitude coordinates into the search bar (or just type them in).

When comparing the two volcano types, shield and composite, it is obvious that although the shield volcanos are more massive, they are far less dangerous to the population than the smaller composite volcanoes. Shield volcanoes produce basaltic lavas that may fountain at the vent, due to gases, but end up flowing passively down the flanks of the volcano.

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